Avoiding Scary Bad Designs: 12 Things Not to Do

Posted On October 26, 2022 By Protolabs

Usually, we like to write about best design practices to heed to help ensure your parts are perfection. In the spirit of all things spooky and Halloween-y, let’s cover some service line specific tips on “what not to do” to avoid downright scary bad parts. Having manufactured hundreds of thousands of parts each year, we’ve seen a few that have sent shivers down our spine! Here’s what to avoid.   

Injection Molding Mishaps

Laying it on Thick

Wall thickness is a key consideration for molded parts. Walls that are too thick are prone to sink, warp, and form internal voids (pockets of air). Never fear though, we’ve got you covered. Adhering to our wall thickness guidelines will help avoid these issues. Note: Keep in mind, this is only a general rule as not all parts may have wall thicknesses at the high and low ends indicated on the chart.

ABS 0.045 - 0.140
Acetal 0.030 - 0.120
Acrylic 0.025 - 0.500
Liquid Crystal Polymer 0.030 - 0.120
Long-fiber reinforced plastics 0.075 - 1.000
Nylon 0.030 - 0.115
Polycarbonate 0.040 - 0.150
Polyester 0.025 - 0.125
Polyethylene 0.030 - 0.200
Polypropylene Sulfide 0.020 - 0.180
Polypropylene 0.025 - 0.150
Polystyrene 0.035 - 0.150
Polyurethane 0.080 - 0.750
Not Implementing Draft and Radii

Draft and radii are vital to a properly designed injection-molded part. Draft helps a part release from a mold with less drag on the part's surface since the material shrinks onto the mold core. Limited draft requires an excessive amount of pressure on the ejection system. That may damage parts and possibly the mold. A good rule of thumb is to apply 1 degree of draft per 1 inch (25.4mm) of cavity depth, but that still may not be enough depending on the material selected and the mold's capabilities. Always check with our design team to ensure the draft you’re slating for your part is sufficient.  

Draft illustration

Draft helps a part release from a mold. Various degrees of draft are illustrated here.

Radii illustration

Sharp corners have high-stress concentration and plastic flow is hindered. In contrast, rounded corners have reduced-stress concentrations and plastic flow is enhanced.

Radii on the other hand, aren’t a necessity for injection molding but should be applied to your part—eliminating sharp corners on your part will improve material flow as well as part integrity. The resin filling the mold cavity flows better around rounded corners. Plastic resin wants to take a path of least resistance to minimize stress on the material and mold. Building in radii also aids in part ejection and reduces the likelihood of the part warping or breaking when it’s removed from the mold.

Box illustration
Deep-Rib Approach: Box designed with walls as rubs. Higher cost to machine and polish cavity. Core-Cavity Approach: Box designed through core-cavity method. Can get to features with bigger, faster cutter. Easier and faster to polish.
Not Coring out or Ribbing

The core and cavity are often referenced as the A and B sides (top and bottom halves) of a mold. A core-cavity approach to part design can save manufacturing time and money and improve the overall part cosmetics. This design technique requires the outside and inside walls to be drafted so they are parallel to one another. This method keeps consistent wall thickness, maintains the part integrity, improves strength and moldability, and decreases overall manufacturing cost.

3D Printing Perils 

Many 3D printing mistakes are manufacturing process specific. Our additive processes are different from each other and come with unique considerations. Here are three printing issues to try to steer clear of. 

Not Uploading a High-Resolution STL File

In some cases, we receive STL files that are low resolution, which can result in coarse faceting (surfaces like gemstones). While a low-resolution file won’t prevent us from manufacturing the part, it could affect its aesthetics. Most CAD modeling software allows you to adjust the resolution in your export settings. We recommend that you ensure your STLs are high resolution, but not so large that they can’t be uploaded or manipulated, about 100MB or less. Another option is to submit a STP/STEP file that we can convert to STL on our end. 

Beyond STL files, we accept native SolidWorks (.sldprt) or ProE (.prt) files as well as solid 3D CAD models from other CAD systems output in IGES (.igs), STEP (.stp), ACIS (.sat) or Parasolid (.x_t or .x_b) format. 

Not Sidestepping Shrink with SLS and MJF

Differential shrink can occur when a part has unequal distribution of material. When one side of a part is much thicker compared to the rest of it, it cools at different rates. The thicker parts cool at slower rates than thin spots, which can result in shrink. If a thick feature is required on the part, we recommend hollowing the feature to a shell of approximately 0.100 in. (2.54mm) to 0.125 in. (3.175mm). If possible, match the overall thickness of your part to the large feature’s shell thickness.

Overhang illustration
When designing for DMLS, be cautious about overhangs—abrupt changes in a part’s geometry. As seen in this example, if the next layer is larger than the previous layer, it will create an overhang.
Not Avoiding Large Overhangs in Metal 3D-Printed Parts

Different from self-supporting angles that offer a smooth slope to a part design, overhangs are abrupt changes in a part’s geometry. DMLS/metal 3D printing is fairly limited in its support of overhangs when compared to other 3D printing technologies such as stereolithography and selective laser sintering. When designing overhangs, it is wise to not push the limits as large overhangs can lead to reduction in a part’s detail and worse, lead to the whole build crashing. Check out this design tip on metal 3D-printed parts, which offers more guidance on how DMLS can create complex, durable, lightweight metal parts. 

CNC Machining Mistakes

Not Avoiding Features that Require Unnecessary Machining

One frequent mistake is designing a part with areas that don’t need machine cutting. Such unnecessary machining adds to your part’s run time—run time that’s a key driver of your final production cost. Consider this example, wherein the design specifies a critical circular geometry needed for the part’s application (see left-side illustration in image at right). It calls for machining the square holes/features in the middle and then cutting away the surrounding material to reveal the finished part. That approach, however, adds significant run time to machine away the remaining material.  

In a simpler design (see right-side illustration at right), the machine simply cuts the part from the block, eliminating the need for additional, wasteful machining of excess material altogether. The design change in this example cuts machine time nearly in half. Keep your design simple to avoid extra run time, pointless machining—and added cost. 

Incorporating Tall, Thin Walls to your Design

Cutting tools used in the CNC machines deflect or bend slightly at machining forces, as does the material being cut. This can result in issues such as an undesirable rippled surface and difficulty meeting part tolerances. Also, the wall could chip, bend, or break. The taller your wall—our maximum is 2 in. (51mm)—the thicker it may need to be to increase the rigidity of the material. A good rule of thumb for walls is a width-to-height ratio of 3:1. Adding some draft to a wall so that it tapers rather than standing vertical, could make machining it easier and leave less leftover material. 

Adding in Small or Raised Text

Your components may require a milled part number, descriptions, or a company logo. Adding text also adds cost. And the smaller the text, the higher the cost. That’s because the very small endmills that cut the text operate at a relatively slower speed, increasing run time on your part and therefore your final cost. On the other hand, cutting larger text goes significantly faster if your part can accommodate it, reducing your cost. Pro tip: If possible, opt for recessed rather than raised text.   

Sheet Metal Woes

Placing Features too Close to Bend Lines
Hole illustration
Be sure to be mindful of where you’ll place holes, tabs, and other features and make sure they aren’t placed too close to a bend, as seen in this example.

A quick way to create difficulties during manufacturing is to place holes, tabs, or other features too close to a bend. So, how close can you get? Just follow the 4T rule. Keep all features at least 4x material thickness away from bend lines. So, if your design tells us to use 0.050 in. (1.27mm) copper, give your feature at least 0.200 in. (5.08mm) of clearance. If you don’t, the part will deform awkwardly in the press brake, and no one wants that. 

Designing Perpendicular Sheet Metal Corners

When you bend sheet metal in a press brake, it doesn’t form a perfect 90-degree angle. Instead, because the tool has a rounded tip, it will add a radius to the bend. If you measure the length of that bent area and divide it by two, you’ll get the bend radius, a figure that is defined by the tool that made it.  

The most common internal bend radius (and our default) is 0.030 in. (0.762mm). An important consideration to remember is that the external bend radius—the one formed on the die side of the press brake toolset—is equal to the material thickness plus the internal bend radius.  Some designers like to get fancy and create different radii for each bend in a part, but if cutting cost is top-of-mind, opt to use the same radius for all the bends.  

Not Including Hardware Specs

Always remember to let your manufacturer know what kind of hardware you want to use by including the details in your top-level assembly information. For example, say you’d like to include a self-clinching nut. Be sure to specify this in your design file to ensure you get what you want placed in the location of the part you expect it.   Minding these 12 “what not to do” tips is a great place to start, but as you begin the manufacturing process, work with our talented applications engineers to help advise on part design. They’re there every step of the way to ensure your parts turn out flawlessly, no matter which manufacturing process you go with.  

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